The present invention relates to a process for the face-side coating of an interior trim part and more particularly, to a process for motor vehicle interior trim parts in which the face side is coated with a cross-linkable curing resin or lacquer and in which the coating of resin or lacquer is subject to an elevated pressure to increase the surface quality, a mold tool with polished surface being laid under pressure and under heat against the face-side surface of the coating and the interior trim part being removed only after drying of the resin or lacquer.
EP 168 026 A2 discloses a process for producing a scratch-resistant, transparent coating of a visor for a motor cycle helmet. A cross-linkable resin or lacquer is applied to at least one side of the visor and is cured. To improve the surface of the coating, the visor with the cured resin or lacquer is subsequently inserted into a mold tool in the form of a plate press and whose tool surfaces opposite the coating are polished. In the mold tool, the surface of the lacquer is acted on with a pressure of, for example, 30 bar, a temperature of approximately 150xc2x0 C. being at the same time present. With the exception of constantly occurring air inclusions in the coating, which inclusions increase the reject rate, this known process provides good results for thermoplastic materials. For interior trim parts such as linings of surfaces, steering wheels, gearshift knobs or the like, which may have, on at least one side, a decorative wood veneer to be coated, however, the known process is too time consuming. Furthermore, the layer thickness of the coating can only be determined within wide limits which, particularly in the case of dimensionally accurate interior trim parts may lead to installation complications.
German Offenlegungsschrift 25 46 466 discloses a conventional process for coating a substrate, such as chipboards, with thermoplastic material. A chipboard and the solid thermoplastic material initially having a cellular structure are inserted in a positionally defined manner into a mold tool. The material is subsequently melted, with the tool closed and a pressure exerted on the thermoplastic material, to form the covering, during which the cellular structure of the thermoplastic material is destroyed. During melting, the upper tool and the lower tool are moved towards one another, so that the contact between the shaping surfaces of the tool and the surface of the coating is not interrupted. By virtue of the varying roughness of the surface of the material to be coated, however, problems occur time and again. For example, in the case of a rough surface, the cellular structure is sometimes only partly destroyed, leading to an impairment of a transparent coating. In the case of very smooth surfaces, although the cellular structure is completely transformed into a homogeneous coating, the layer thickness of the coating in this case is too high in some circumstances, since the mold tool cannot be completely closed. With both surfaces, for example in the case of wood surfaces having air inclusions, system-dependent air inclusions occur repeatedly in the coating. For this reason, the conventional coating process is also unsuitable for transparent coatings because of such defect sources. Furthermore, this conventional process is also very time consuming, since, in addition to the substrate, the thermoplastic material also has to be inserted into the tool, with the thermoplastic material having previously to be machined to the dimensions appropriate for insertion. Cellular-structure starting material is also expensive.
An object of the present invention is to provide an improved coating process such that, with a minimum reject rate caused by a visible inclusion of air, a coating, which is as transparent as possible and largely free at least of visible bubbles, of an interior trim part face-side surface is possible with relatively little time consumption.
The foregoing object has been achieved according to the present invention by a process in which the interior trim part is inserted, in uncoated form, in a defined position in a diecasting or injection-molding mold tool, the resin or the lacquer being applied to the surface, to be coated, of the interior trim part in the manner of an injection molding operation or diecasting operation, in that the resin or the lacquer is applied in the closed mold tool with the required overall thickness in a single operation and, in that, during the entire curing time, despite shrinkage due to curing, a temporally uniform pressure is exerted on the resin or lacquer, which is initially liquid and subsequently curing in the mold tool, said pressure being chosen such that any air present in the liquid resin or lacquer is dissolved.
By virtue of the new process according to the present invention, including the surface forming and the curing of the resin or lacquer in a single tool, the production time needed for a transparently coated interior trim part is reduced and it is possible to remove the finally processed workpiece from the mold tool. Furthermore, any interior trim part can therefore be produced with high precision within predeterminable dimensions. The reject rate as a result of air bubbles visibly dissolved in the coating is, at the same time, reduced because this air largely goes into solution at the prevailing pressure.